Mechanisms of Past Climate Change (16:107:553) Fall 2007 Ice Ages and Changes in Earth’s Orbit

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Topic Outline Introduction to the Quaternary Oxygen isotopes as an indicator of ice volume Temporal variations in ice volume Periodic changes in Earth’s orbit Relationship between orbital changes and variations in ice volume

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Topic Outline Introduction to the Quaternary Oxygen isotopes as an indicator of ice volume Temporal variations in ice volume Periodic changes in Earth’s orbit Relationship between orbital changes and variations in ice volume

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Geologic Time Scale

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Geologic Time Scale

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Geologic Time Scale

Mechanisms of Past Climate Change (16:107:553) Fall 2007 The Quaternary Period In the first half of the 19 th century, Louis Agassiz argued that widespread glaciation was the explanation for various unusual geologic features in much of North America and Europe. A lengthy scientific debate ensued, but the evidence for a number of continental glaciations gradually became accepted.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Moraines As a glacier advances, its leading edge acts like the blade of a bulldozer, pushing rock and debris in advance. These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Moraines As a glacier advances, its leading edge acts like the blade of a bulldozer, pushing rock and debris in advance. These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Moraines As a glacier advances, its leading edge acts like the blade of a bulldozer, pushing rock and debris in advance. These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 The Surface of the Ice Age Earth

Mechanisms of Past Climate Change (16:107:553) Fall 2007 LGM Ice Extent in the Northeastern United States Moraines from earlier glaciations are most often destroyed by subsequent glaciations, so moraines are generally evidence of the most recent glacial advance.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Topic Outline Introduction to the Quaternary Oxygen isotopes as an indicator of ice volume Temporal variations in ice volume Periodic changes in Earth’s orbit Relationship between orbital changes and variations in ice volume

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Oxygen Isotopes A small fraction of water molecules contain the heavy isotope 18 O instead of 16 O. 18 O/ 16 O ≈ 1/500 This ratio is not constant, but varies over a range of several percent. Vapor pressure of H 2 18 O is lower than that of H 2 16 O, thus the latter is more easily evaporated.

Mechanisms of Past Climate Change (16:107:553) Fall 2007  18 O As water vapor is transported poleward in the hydrologic cycle, each cycle of evaporation and condensation lowers the ratio of H 2 18 O to H 2 16 O, in a process called fractionation. This ratio is expressed as  18 O.

Mechanisms of Past Climate Change (16:107:553) Fall 2007  18 O vs. Temperature As a consequence of fractionation,  18 O in precipitation decreases with decreasing temperature. Ice sheets have very low  18 O values. Observed  18 O in average annual precipitation as a function of mean annual air temperature (Dansgaard 1964). Note that all the points in this graph are for high latitudes (>45°). (From Broecker 2002)

Mechanisms of Past Climate Change (16:107:553) Fall 2007  18 O and Global Ice Volume As ice sheets grow, the water removed from the ocean has lower  18 O than the water that remains. Thus the  18 O value of sea water in the global ocean is linearly correlated with ice volume (larger  18 O → larger ice sheets). A time series of global ocean  18 O is equivalent to a time series of ice volume.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Obtaining a  18 O Time Series Microscopic marine organisms called foraminifera incorporate oxygen into their shells in the form of CaCO 3. When these organisms die, their shells fall to the sea floor and are deposited in deep sea sediments.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Obtaining Sediment Cores As sediments accumulate, the properties of the overlying ocean are recorded sequentially. Sediment cores are obtained by drilling into the sea floor.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Obtaining Sediment Cores The sediments are analyzed, using both chemical and visual analysis. To produce a time series of ocean properties, a chronology or “age model” must be developed.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Chronology A simple age model can be obtained by assuming a constant accumulation rate. Reversals in Earth’s magnetic field can be used for benchmarks. Magnetic reversals have been radiometrically dated.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Chronology A simple age model can be obtained by assuming a constant accumulation rate. Reversals in Earth’s magnetic field can be used for benchmarks. Magnetic reversals have been radiometrically dated. Brunhes- Matuyama magnetic reversal

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Other Sources of  18 O Variation Complicating factor: Changes in ice volume are the largest contributor to  18 O variations, but they are not the only one. Regions of the ocean in which evaporation exceeds precipitation are enriched in  18 O, and vice versa. Isotope separation between water oxygen and shell oxygen depends on temperature.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Solution Changes in  18 O driven by variations in P-E are largest near the ocean surface, so  18 O from benthic (i.e., deep dwelling) forams are more representative of global ocean  18 O. The Pacific deep ocean temperature is very close to freezing, so it could not have been much colder during glacial periods.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Topic Outline Introduction to the Quaternary Oxygen isotopes as an indicator of ice volume Temporal variations in ice volume Periodic changes in Earth’s orbit Relationship between orbital changes and variations in ice volume

Mechanisms of Past Climate Change (16:107:553) Fall 2007

Mechanisms of Past Climate Change (16:107:553) Fall 2007

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Topic Outline Introduction to the Quaternary Oxygen isotopes as an indicator of ice volume Temporal variations in ice volume Periodic changes in Earth’s orbit Relationship between orbital changes and variations in ice volume

Mechanisms of Past Climate Change (16:107:553) Fall 2007

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Earth’s Orbit Can Vary

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Earth’s Orbit Can Vary

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Earth’s Orbit Can Vary

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Eccentricity Eccentricity = (distance from focus to center) / (length of semimajor axis) Eccentricity of Earth’s orbit varies from 0 to 0.05, with 100-kyr, 400- kyr and 2 Myr periodicities.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Eccentricity

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Obliquity Obliquity (i.e., tilt) of Earth’s axis varies from 22° to 24.5°, with a 41-kyr periodicity.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Obliquity

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Precession The Earth’s axis precesses, or wobbles, with periodicities of 19 kyr and 23 kyr.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Precession

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Astronomical Theory of Ice Ages In 1842, J. Adhémar suggested that slow variations in Earth’s orbit could be responsible for climatic changes by altering the lengths of the seasons. In 1875, J. Croll hypothesized that orbital variations might lead to substantial changes in climate. (Colder winters → larger snow cover → glaciation)

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Renewed interest in orbital forcing of glacial cycles occurred when M. Milankovitch (1941) computed long-term variations in insolation. Milankovitch believed that cold summers led to glaciation by allowing snow to survive into the next year. Milankovitch

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Three Conceptual Models of Orbital Effects on Glacial Cycles

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Temporal Variation of Orbital Parameters Eccentricity: Relatively low for the past 60 kyr. Obliquity: Variations have been quite regular; current value of 23.5° near mean. Precession: Perihelion currently occurs near NH winter solstice.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 In the N. Hemisphere, the effects of tilt and distance act in opposite directions, although tilt dominates. In the S. Hemisphere, the effects of tilt and distance are in phase, yielding an amplified seasonal cycle of insolation.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Insolation at 65°N High latitude summer insolation (June, 65°N) has been regarded as an index of orbital forcing of glaciation. (This is the original Milankovitch hypothesis: Cool summers are beneficial to ice growth.) Note that the effects of precession are modulated by eccentricity. For low summer insolation: Aphelion in summer (esp. with high eccentricity), low obliquity.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Topic Outline Introduction to the Quaternary Oxygen isotopes as an indicator of ice volume Temporal variations in ice volume Periodic changes in Earth’s orbit Relationship between orbital changes and variations in ice volume

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Turning Point for Astronomical Theory of Ice Ages Hays, J. D., J. Imbrie, and N. J. Shackleton, 1976: Variations in the Earth’s orbit: Pacemaker of the ice ages. Science, 194, “It is concluded that changes in the earth’s orbital geometry are the fundamental cause of the succession of Quaternary ice ages.”

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Peaks in  18 O Spectrum Correspond to Orbital Frequencies Variance spectra for marine oxygen isotopes for the last 700 kyr (lower curve) compared with spectra for Earth’s orbital parameters (Imbrie,1985). (From Broecker, 2002)

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Spectral Analysis of SPECMAP Stacked  18 O Record Distinct peaks in ice volume record at orbital frequencies are present. These peaks are robust, even when more powerful spectral methods are used.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 The 100-kyr Problem

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Model 1: Calder (1974) V = ice volume i = summer insolation at 65 ° N i 0 = insolation threshold k = k A (accumulation) if i < i 0 k = k M (melting) if i > i 0

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Model 2: Imbrie and Imbrie (1980) Written in dimensionless form (i.e., variables are divided by a scaling value) V = ice volume V i = equil. ice volume at insolation i i = summer insolation at 65 ° N  =  M if V > i (melting)  =  A otherwise

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Model 3: Paillard (1998)

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Model 3: Paillard (1998) Very good agreement with record, both in time and frequency domain. Weakness: Highly nonlinear, with a number of adjustable parameters.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Ice Core Paleoclimatology As snow falls on very cold glaciers or ice sheets and gradually is converted to ice, air is trapped in bubbles. This “fossil air” can be chemically analyzed to determine past atmospheric composition. Other paleoclimatic proxies (isotopes, dust, acidity) can also be determined from the ice, providing information about temperature, sulfate aerosols, precipitation.

Mechanisms of Past Climate Change (16:107:553) Fall 2007 Multiproxy Analysis of Glacial Cycles Glacial-interglacial cycles are evident in a variety of paleoclimatic and paleoceanographic proxies. The shapes of the cycles vary somewhat among the different proxies. Glacial-interglacial variations in atmospheric CO 2 concentration are substantial. (But what causes them?) There are uncertainties in time scales.